Explain briefly the separation technique of fractional distillation, with reference to a particular example.

Fractional distillation is used to separate crude oil into its constituent fractions (e.g. petrol, naphtha etc. - names not required)Crude oil is heated to a high temperature and most is vaporised/evaporated, heavier molecules do not boil. Oil vapours/gases rise up the tower and condense into liquid at different points, this depends on their boiling point. Lighter molecules rise further up the tower than heavier molecules. The oil is therefore separated into fractions containing molecules with similar length hydrocarbons. Overall there is a temperature gradient up the tower.